Create multiple bundles for production

[rtibbles]

[rtibbles]

Allow modern browsers to benefit from ES2015+ code natively, reduce bundle size, and enhance performance.

Following advice in this article: https://philipwalton.com/articles/deploying-es2015-code-in-production-today/

Will require updating the kolibri-tools build pipeline, to produce multiple bundles in production.

It would also require updates to the webpack script tag bootstrapping in order to allow loading of either the new mjs bundle and the old js bundle.